Production of para-xylene



Patented Dec. lb, 1963 3,113,973 PRQDUQTEGN (El EPARA-XYLENE Michael J. Derrig, Verona, and Arthur C. Whitaker, Pittsburgh, Pa, asslgnors to Gull? Research 8. Eevelop ment Company, Pittsburgh, Pa, a corporation of Delaware Filed Sept. 13, 1955, Ser. N 5.34pm 9 Qlaims. (Cl. 266-4563) This invention relates to a process for the production of aromatic hydrocarbons and more particularly to the production of p-xylene.

Processes for reforming petroleum hydrocarbons boiling in the gasoline boiling range have greatly increased the quantity of aromatic hydrocarbons produced in the refining of petroleum fractions. Among the aromatic hydrocarbons produced from petroleum fractions are the xylene isomers which are usually present in approximately the equilibrium ratio of about 2 parts or? o-xylene, 2 parts p-xyiene and 5 parts m-xylene. In spite of the greatly increased supply of aromatic hydrocarbons resulting from reforming operations, the demand for p-xylene has increased to such an extent that it is desirable to increase the yield of p-xylene from petroleum fractions still further.

In the process usually employed for the recovery of pure xylene isomers, o-xylene is separated from the mand p-xyle-nes by a precision distillation; however, the boiling points of mand p-Xylenes are too close to allow separation by distillation. The resulting mixture of mand p-xylenes is fractionally crystall zed to separate crystals of substantially pure p-xylcne. A low melting point eutectic mixture of mand p-xylenes containing about 12 percent p-xylene prevents recovery of all or the p-xylene by crystallization. The mother liquor, which is rich in m-xylene, obtained from the crystallization can be isomerized to convert part of the rn-Xylene to p-xylene, but the isomerization of m-xylene yields a reaction product having at best the equilibrium ratio of pto m-xylene.

in this invention, naphthenic compounds containing 8 carbon atoms per molecule are isomerized to give a p-xylene precursor in concentrations higher than the concentration of p-xylene in an equilibrium mixture of Xylenes, and the p-xylene precursor is dehydrogcnated to form p-xylene. The p-Xylene thus formed can be separated from other compounds present in the dehydrogenated. product by conventional methods such as those described in the preceding paragraph. In a preferred embodiment of the process a charge stock consisting principally of m-xylene is hydrogenated to dimethylcyclohexane and the dimcthycyclohexane iscmerized. An especially high yield of p-xylene can be obtained if the isomerizate is fractionally distilled a precision distillation to form an overhead product consisting of a mixture of trans-1,4 dimethylcyclohexane, cisl,3 dimethylcyclohexane and 1,1 dimethylcyclohexane as an overhead product. The overhead product from the distillation is then dehydrogenated to yield p-xylene which is separated from other xylene isomers.

The single FIGURE in the drawing is diagrammatic representation of the procedural steps employed in a preferred embodiment of this invention.

The feed stock for the process of this invention can be any hydrocarbon fraction which includes cyclic hydrocarbons having 8 carbon atoms per molecule. For example, a feed stock for a reforming operation such as a platformer feed stock can be used. contains naphtlienic and parafimic hydrocarbons, those compounds will be substantially unailected in the hydrogenation step, which is for converting xylenes to dim-ethylcyclohexanes. In general, it is desirable to have low If the feed stock concentrations of aromatic hydrocarbons in the isomeri- Zation charge stock to reduce the consumption of isom-, erization catalyst and the sludge formation in theisomeri zation step.

A preferred charge stock is a mixture of C-8 aromatic hydrocarbons which may be obtained from a reforming process, for example, Platforming, combined with processes such as Udex or Arosorb processes for the recovery of the aromatic hydrocarbons. The Platform ng, Udex and Arosorb processes are well known commercial processes. Since the primary object of this invention is the production of p-xylene for recovery of that isomer substantially free of other xylene isomers, apparatus for separating pxylene from m-xylene ordinarily would be available at a plant or refinery using this invention. Hence, a stream of m-xylene from which the o-xylene has been removed by fractional distillation and p-xylene has been removed by fractional crystallization is an especially desirable feed stock. The m-xylene will ordinarily contain about 12 percent p-xylene and may also contain ethylbenzene. Ortho-xylene can also be used as a charge stock.

The hydrogenation step is only sufiiciently severe to convert aromatic hydrocarbons to naphthenic hydrocarbon-s; hence, it may easily be carried to substantial completion. Many of the well known catalysts used in conventional hydrocarbon hydrogenation processes can be employed at hydrogenation conditions. Raney nickel is a preferred hydrogenation catalyst and can be employed at mild conditions such as at temperatures of 200 C. and a pressure of 1500 pounds per square inch. Examples of other catalysts which can be employed are nickel, copper chromite, nickel oxide, molybdenum sulfide, tungsten sulfide, cobalt molybdate and chromia, either alone or supported on a suitable carrier such as alumina. The temperature, pressure and time used in the hydrogenation are interdependent and also depend upon the hydrogenation catalyst used. In general, the temperature will be in the range of C. to 375 C., the pressure will be 500 pounds per square inch, or higher, and the time less than about 3 hours.

At the mild hydrogenation conditions employed, isomerization of the feed stock will ordinarily be slight, and, particularly when the feed stock is predominantly mxylene, any isomerization that does occur may be beneficial. The ease with which the aromatic hydrocarbons can be hydrogenated will cause the concentration of the aromatic hydrocarbons in the hydrogenated product to be small in most instances. However, it may be desirable to strip the xylenes or other aromatic hydrocarbons from the hydrogenated material prior to the isomerization step. The aromatic hydrocarbons can be stripped by the use of selective solvents or by passing the hydrogenated hydrocarbons over silica gel.

The naphthenic hydrocarbons from the hydrogenation step are isomerized, preferably at a low temperature in the presence of a highly active isomerization catalyst. A low temperature, below about C., and preferably between about 30 C. and 90 C., favors an equilibrium in which the concentration of 1,4 dimethylcyclohexanes is bir h and there is a high ratio of trans-1,4 dimethylcyclohexane to cis-l,4 dimethylcyclohexane. The high ratio of trans-1,4 dimethylcyclohexane to cis-l,4 dimethylcyclohexane is of value in increasing the concentration of pxylene precursors in the manner to be described. The isoi erization is preferably carried out in the presence of an aluminum chloride catalyst in concentrations of about 1 to 30 percent, which allows reaction times as short as one hour with satisfactory yields of p-xylene precursors and reaction times as short as about 3 hours for a substantially complete isomcrization to an equilibrium composition of dimethylcyclohexanes. A low temperature is especially valuable in reducing sludge formation in isomerization in the presence of aluminum chloride. Other catalysts, such as hydrogen fluoride and boron trifluoride, can also be employed; however, those catalysts require higher temperatures with resultant less favorable equilibrium compositions and longer reaction times.

The isomerization product, designated as the isomerizate, contains the following mixture of isomers of dimethylcyclohexane.

Isomer of dimethylcyclohexane: Boiling point, C.

Trans-1,4 119.35 1,1 119.54 Cis-1,3 120.09 Trans-1,2 123.42 Cis-1,4 124.32 Trans-1,3 124.45 Cis-1,2 129.73

The ratio of trans-1,4 dimethylcyclohexane to cis-1,4 dimethylcyclohexane in the isomerizate is substantially higher than the ratio of cis-l,3 dimethylcyclohexane to trans-1,3 dimethylcyclohexane. The approximately 33 C. difference in the boiling point of the cis-1,3 dimethylcyclohexane and trans-1,2 dirnethylcyclohexane permits an efficient separation to be made between the first three isomers listed and the last four in a precision distillation such as can be obtained, for example, in a 70 plate Stedman column. This fractionation produces an overhead product having a high, and a bottoms product having a low, concentration of 1,4 dimethylcyclohexanes. If desired, the distillation can be made to leave a portion of the cis-1,3 dirnethylcyclohexane in the bottoms product, or a more efficient distillation can be used, to increase the concentration of trans-1,4 dimethylcyclohexane in the overhead product.

Either the total isomerizate, or the overhead product from the distillation of the isomerizate if a distillation step is employed, is dehydrogenated in a conventional dehydrogenation process at conditions to avoid isomerization of the resulting aromatic hydrocarbons. The dehydrogenation can be accomplished, for example, in the presence of a noble metal, such as platinum or palladium, dehydrogenation catalyst. A preferred dehydrogenation catalyst is platinum, which may be deposited on a support. Suitable supports are alumina, silica, and deactivated silicaalumina. If a platinum dehydrogenation catalyst is used, the isomerizate can be dehydrogenated at temperatures of about 250 to 400 C., for example, about 325 C.

Other well known dehydrogenation catalysts such as chromium oxide supported on alumina are suitable for the dehydrogenation, but require temperatures in the range of about 500.to 600 C. Molybdena, preferably supported on alumina, and cobalt molybdate can also be used. The dehydrogenation is preferably accomplished at low pressures such as atmospheric pressure.

The invention may be more clearly understood by reference to the attached drawing in which preferred embodiments of the process for the production of p-xylene from m-xylene are illustrated diagrammatically. The m-xylene charge stock is introduced through a feed line 19 to a hydrogenated step 12, in which the m-xylene is substantially completely hydrogenated to form 1,3 dimethylcyclohexanes. .Hydrogen for the hydrogenation is introduced through a supply line 14 which joins feed line 10.

The dimethylcyclohexanes are delivered from the hydrogenation step 112 through a line 16 to an aromatichydrocarbons removal step 18. Ordinarily the aromatic hydrocarbons are substantially completely hydrogenated; hence, the quantity that must be removed in step 18 will be small, for example, about one percent or less of the total hydrogenated product. The aromatic hydrocarbons removed in step 18 are preferably recycled through a line 20 to the feed line 10. If desired, the aromatic hydro- 4 carbons stripped from the hydrogenated product in step 13 can be removed from the system through line 22.

The substantially aromatic-hydrocarbon-free dimethylcyclohexanes are delivered from aromatic-hydrocarbons removal step 18 through line to an isomerization step 26. The isomerizate from step 26 is delivered through a line 23 to a precision distillation step 35 in which the 1,1 dimethylcyclohexane, trans-1,4 dimethylcyclohexane and sis-1,3 dimethylcyclohexane are removed as an overhead product, and delivered through a line 32 to a dehydrogenation step 34. The bottoms product firom the distillation step 3d, consisting of cis-l,4 dimethylcyclohexane, trans-l,3 dimethylcyclohexane, cis-1,2 dimethylcyclohexane and tran -1,2 dimethylcyclohexane, can be recycled through line 3 6 to line 24 for another pass through the iso-merization step 25 or discharged from the system through a line 33.

The dehydrogenation step 3 d is accomplished at conventional dehydrogenation conditions to convert the dimethylcyclohexanes to xylenes. Hydrogen liberated from step 34 is recycled through a line to the hydrogenation step 112.

The Xylenes produced in the dehydrogenation step 34 are delivered through a line d2 to a fractional crystallization process 44 for the separation or" mand p-Xylene. The isornerization or" the naphthenic compounds combined with the precision distillation allows the production of 21 Xylene stream having a concentration of approximately 35 percent p-xylene. m-Xylene separated from the p-Xylene in the fractional crystallization is recycled through a line 45 for additional passes through the proc- CSS.

The following examples illustrate the improved yield of p-Xylene made possible by this invention.

EXAMPLE I A charge stock of a hydrocarbon fraction containing 2.1 percent ethylbenzene, 1.1 percent o-xylene, 1.6 percent p-Xylene and the remainder m-Xylene, mixed with Si) percent of its weight of aluminum chloride, was agitated at a temperature of 55 C. for a period of 2 hours. The aluminum chloride and reaction products were separated and the reaction products analyzed to determine the concentration of the individual hydrocarbon compounds.

EXAMPLE II A sample of the charge stock employed in Example I admixed with 30 percent of its Weight aluminum chloride was agitated at a temperature of C. for a period of 2 hours. The reaction product was separated from the aluminum chloride and analyzed.

EXAMPLE III A sample of the charge stock of Example I was mixed with 30 percent of its weight of aluminum chloride and agitated at a temperature of 85 C. for 4 hours. The reaction product was separated from the aluminum chloride and analyzed.

EXAMPLE IV A sample of the charge stock employed in Example I was hydrogenated at 309 F. and 1500 pounds per square inch gauge in the presence of 10 percent by weight of Raney nickel. Approximately 0.5 percent of the m- Xy'lene was not converted to naphthenes and was removed from the reaction products by treatment with silica gel. The dimethylcyclohexanes formed by the hydrogenation were mixed with 30 percent of their weight of aluminum chloride and agitated at a temperature of 65 C. for a period of 3 hours. A sample of the reaction products was analyzed to determine the concentration of each of the isomers of dimethylcyclohexane. A separate sample of the isomerizate was dehydrogenated at 325 C. by passing it over platinum on activated car hon at aliquid hourly space velocity of 0.16. A conversion of the naphthenes of 96-97 percent was obtained Without appreciable isomerization. The dehydrogenated reaction product was analyzed to determine the concen- 6 It will be seen froin Table I that the hydrogenation of the m-Xylene to dimethylcyclohexanes prior to isornerization allows the concentration of the 1,4 isomer in the isornerizate to be increased to approximately 26 percent from tration of individual xylene isomers and ethyl benzene. 5 the 22 percent obtained upon the isomerization of rn-xy- EXAMPLE V lene. When a preferred feed stock, the mother liquor from a fractional crystallization process for the separa- A sample of the charge stock employed in Example I tion of rnand p-Xylcnes, is employed, direct isomerizawas treated 1n the manner described for Example IV tion of Iii-xylene to porylene only gives a not gain of about with the exception that a small amount of arsenious 1 10 precent p-Xylene. The hydrogenation of rnxylene chloride Was added to the aluminum chloride. prior to lSOIllfil'lZEll-lOll allows a net gain in p-xylene of EXAMPLE VI about 14 percent. When the naphthene isomerizate is Efractionally distilled prior to dehydrogenation, as in EX- A Sample 9 the charge Stock p y 111 Example I arnples VH1 and 1X, an increase of about 2.2 percent in W (mated 1I1 the manner descnbsd Example IV 15 the concentration of p-Xylene is obtained. Even higher Wllh the @XWPLIOYI thatfi Small l f 0f fiffmildehydfi gains are possible with more efficient fractionation and was added to the aluminum chloride isomerrzatron catat i f h wt jm to have a portion f the (354,3 y dimethylcyclohexane in the bottoms product.

A Sample of m charge stack employed in Examnle I -1. A process for the production p-Xylene comprising was treated in the manner described for Example IV hYdmgFDatmg Iflxylenfi to form dlmethylcyclongxanes with the exception that the naphtnene fraction was agicfmtactmg the dlmethyicyqlohexanes with aluminum c5110 tated with the aluminum chloride isomerization catalyst ride Tang: of to 1380 for a period of 61/2 hours C. to form an isomerizate containing a high concentration of 1,4 diniethylcycl ohexanes, rand dehydrogenating EXAMPLE VII! the isomerizate to the corresponding isomers of Xylene.

A sample of a char-2e stock ernnloyed in the other ex- A P509668 the Pmdmfim of mixture 05 s amplgs was hydmgenated in h manner described f aromatic hydrocarbons having a p-Xylene concentration Example 1V and isomerized at the conditions d ib d higher than the concentration of p-Xylene in an equilibrium for Example IV. The isomerizate was distilled in a premvfimre P a aromatic y f l comprising C011- cision f ti ti column senarate 4 dimethyp tecting dimethylcycloheirane with aluminum chloride at a cyclohexane, trans-1,4 dimethylcyclohexane and 1,1 di- 'tempammne mime Tm 3B of to 1500 C? {0 an methylcyclohwane as an Overhead product. A Sample isornerizate having a high concentration of 1,4 dimethylof the overhead product was analyzed to determine the cyolchexiawsr than comzlwnlg 150mproportions of the different nanhthene isomers. The hydmgmatwn Catalyst 1,4 Y Y averhsad product was dehydmgsnated in the presence 0f ones to p-Xylene, the contact or the drmethylcyclonexanes a piatinum dahydmgemfion Catalyst in the manner and dehydrogenation catalyst being at conditions to avoid scribed for Exam-IP15 substantial lsomemziatron of the p-xylene.

V n 3. A process as set forth in claim 2 in which the de- EAAMPLM IX 40 hydrogenation catalyst is a platinum catalyst and the tem- A sample f th Charge t k emplgygd i E l 1 perat-ure at which hhe iso rn erizate is contacted with the was treated in the manner described for Example VH1 dehydrogenationCatalyst15in Th6 fangs 015 to with the exception that following the isomerization re- Process for P Q of a mlXiufe of 0 action in the presence of aluminum chloride, the reaction hydfosalbons hflvlng P y l l' product was fl d fg 16 hours t a temperature f 4 higher than the concentratron of p-Xyilene in an equilibrium 119 C. in the presence of a trace of aluminum chloride. mixture o 1116 3 aromatic hydfocfifboils ficmpfising The results or" the experimental runs are set forth in tam g 1, i hylcy l hex ncs h a uminum h ri Table I. at a temperature in the range of about 30 to 150 C. to

Tobie I Example N 0. Charge Stock Hydrogenated Isomerization:

Temperature, O. 85 65 65 65 65 65 1 65 Time, hours 2 2 4 3 3 6. 5 3 1 3 A1013, percent of Charge Stock -1 30 30 30 3O 2 30 3 30 30 3O 30 Isomerizato, Dimethylcyclohexanc, per

cent by vol.:

1,1Isomer 4.4 5.4 7.1 6.7 ore-1,2 i omer 0.3 5.1 0.9 2.4 Trans-1,2 Isomer 11.2 10.6 9.8 9.6 Cis-1,3 Isomer 43.2 42.6 40.8 39.8 Trans-1,3 Isomer 5.6 7.1 7.6 11.4 Ois-1,4 Isomcr 1.2 1.6 1.0 3.4 Trans-1,4 isomer. 28.1 27.6 26.8 26.7 Overhead Fraction, Dimethylcyclohexane, percent by Vol.2

Cis-1,31somer 1 57.6 56.9 Trans-1,4 Isornor 36.2 37.8 1,1 Isomen. 6.2 5. 3 Dehydrogenated Product Analysis, per

cent by vol.:

Ethylbenzene 2.5 2.6 1.0 0.6 2.2 2.6

.4 3.5 17.2 18.2 10.4 10.7 8.8 0.0 95.7 81.7 58.6 58.7 58.2 55.1 55.0 60.3 p'xylene 1.4 7.0 21.5 22.1 26.4 26.6 25.9 34.4 1,1 dimethy yclohexan 1.5 4.4 5.4 7.1 5.3

1 Refluxed at C. for 16 hours after reaction period. 2 Promoted with A501 3 Promoted with ECHO.

7. form an isomeriziate containing a high concentration of 1,4 dimethylcyol'ohex anes, separating the isomerizate from the aluminum chloride, distilling the thus separated isom erizate to form an overhead product consisting prin cipally of trans-'1,4 dimethylcyclohexane and cis-l,3 dimethylcyclohexane in a precision distillation, and passing the overhead product in contact with a dehydrogenation catalyst at conditions to convert 1,4 d-imethylcyclohexanes to p-xylene and to avoid substantial iscrnerization of the p-xylene.

5. A process for the preparation of a mixture of C31 aromatic hydrocarbons having a p-xylene concentration higher than the concentration of p-xylene in an equilibrium; mixture of i re C aromatic hydrocarbons comprising contacting dimethylcyclohexane with an active isomerization. catalyst at a low temperature to form an isomerizate cont aining a high concentration of 1,4 dime-thylcyclohexanes,,

eparatin g the isomerizate from the isomerizat-ion catalyst, distilling [the isomerizate to separate an overhead product; consisting principally of trans-1,4 dimethylcyclohexane and cis-1,3 dime-thylcyclohexane from trans-l,3 dimethyl-- cyclch enane and ci-1,4 dimethylcyclohex-ane, recycling the bottoms product from the distillation through the isomerization step, and dehydrogenating the overhead product to convert trans-1,4 dimethylcyclohex ane to p-xylene..

6. A process for the production of p-xylene comprising passing a mixture of m-xylene and hydrogen in contact with Raney nickel :at a temperature of about 200 C. and a pressure of about 1500 pounds per square inch to con- "vert Il'i-XYiIlG to dimethylcyclohexaues, contacting the dimethylrcyclo'hexanes with aluminum chloride at a temperature in therange of about 30 to 150 C. to form an isomerizate containing a high concentration of 1,4 dimethylcyclohexanes, and passing the 1,4 dimethylcyclohexanes in contact wi r platinum supported on activated carbon at a temperature of about 325 C. to dehydrogenate the 1,4 dimethylcyclohexanes and thereby form p-xylene.

7. A process for the production of p-xylene from an isomer of xylene other than p-xylene comprising hydrogenating said isomer to form dimethylcyclohexane, then contacting the thus formed dimethylcyolohexane with an isomerization catalyst to form a mixture of isomers of dirnethylcyclohexane having a concentration of 1,4-dimethyicyolohexane higher than the concentration of p- Xylene in an equilibrium mixture of xylenes, distilling the mixture of isomers of dimethylcyclohexane to separate an overhead product consisting principally of trans-1,4- di-methylcyclohexane and cis-l,3 dimethylcycloliexane from cis-1,4-dimethylcyolohexane and trans-1,3-dimethylcyclohex ane, and passing the overhead product in contact with a dehydrogenation catalyst at conditions to convert 1,4dimethylcyc1ohexane in said overhead product to p-xylene.

8. A process for the production of p-xylene from an isomer of xylene other than p-xylene comprising hydrogenating said isomer to form dimethylcyclohexane, then contacting the dimethylcyclohexane with an isomerization catalyst at a tempecature below about C. to form a mixture of isomers of dimethylcyclohexane, distilling the mixture of isomers of dimethylcyclo-hexane to separate an overhead product consisting principally of trans-1,4-dimethylcyclohexane and cis-1,3 dimethylcyclohexaue, from cis-1,4-dimethylcyclohexane and trans-1,3-dimethylcyclohexane, and passing the overhead product in contact With a de hydrogen-ationcatalyst at conditions to convert 1,4-dimethylcyclohexane'in said overhead product to pxylene.

9. A process for the production of p-xyleue from an isomer of xylene other than p-xylene comprising hydrogena-ting said isomer to form dimethylcyclohex ane, contacting the thus formed di-methylcyolohexane with aluminum chloride at a temperature below about 150 C. to form an isomerizate containing 1,4-dime=thylcycylo* hexane, md dehydrogenating the isomerizate at condition-s whereby the 1,4-di-methyicyclohexane is dehydrogenated to p-xylene.

References Cited in the file of this patent UNITED STATES PATENTS 2,282,231 Mattox May 5, 1942 2,389,801 Mattox NOVJ27, 1945 2,422,798 Pines June 24, 1947 2,532,276 Birch et a1 Dec. 5, 1950 2,632,779 Pfennig Mar. 24, 1953 2,741,646 Clark Apr. 10, 1956 2,784,241 Helm Mar. 5, 1957 2,885,451 Linn May 5, 1959 OTHER REFERENCES Thomas: Anhydrous Aluminum Chloride in Organic Chemistry (New York: Reinhold Pub. Corp, 194 1). Page 823 only needed.

Berkman et al.: Catalysis (New York: Reinhold Publ. Corp 1940). Page 989 only needed.

ind. and Eng. Chem, vol. 47, No. 4, pages 770-773 (page 770 only needed).

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 113,978 December 10, 1963 Michael J Derrig et a1.

rror appears in the above numbered pat- It is hereby certified that e the said Letters Patent should read as ent requiring correction and that corrected below.

Column 3. line 63, for "hydrogenated" read hydrogenation column 7, line 19, before "isomerizate" insert thus separated Signed and sealed this 14th day of April 1964.

(SEAL) EDWARD J BRENNER Attest:

ERNEST W. SWIDER Commissioner of Patents Attesting Officer 

1. A PROCESS FOR THE PRODUCTION OF P-XYLENE COMPRISING HYDROGENATING M-XYLENE TO FORM DIMETHYLCYCLOHEXANES, CONTACTING THE DIMETHYLCYCLOHEXANES WITH ALUMINUM CHLORIDE AT A TEMPERATURE IN THE RANGE OF ABOUT 30* TO 150* C. TO FORM AN ISOMERIZATE CONTAINING A HIGH CONCENTRA- 